[dev.typeparams] go/parser: error for type instances without ParseTypeParams

It should be an invariant that the parser does not produce ast.CallExprs
with Brackets == true unless parsing with ParseTypeParams.

Fix the one case where this invariant was violated, and add a test for
errors produced in valid generic code when ParseTypeParams is unset. We
did have some coverage of errors in short_test.go, but I find them to be
easier to read in a testdata file and would like to gradually migrate
them there.

Change-Id: If2d174377087daa1b820cabc2b5bf8bcb0b39d8e
Reviewed-on: https://go-review.googlesource.com/c/go/+/284192
Run-TryBot: Robert Findley <rfindley@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>
Trust: Robert Griesemer <gri@golang.org>
Trust: Robert Findley <rfindley@google.com>

1 files changed, 7 insertions, 0 deletions

diff --git a/src/go/parser/parser.go b/src/go/parser/parser.go
index 24e84d5103..ccbcef8f26 100644
--- a/src/go/parser/parser.go
+++ b/src/go/parser/parser.go
@@ -1494,6 +1494,7 @@ func (p *parser) parseIndexOrSliceOrInstance(x ast.Expr) ast.Expr {
 	var args []ast.Expr
 	var index [N]ast.Expr
 	var colons [N - 1]token.Pos
+	var firstComma token.Pos
 	if p.tok != token.COLON {
 		// We can't know if we have an index expression or a type instantiation;
 		// so even if we see a (named) type we are not going to be in type context.
@@ -1512,6 +1513,7 @@ func (p *parser) parseIndexOrSliceOrInstance(x ast.Expr) ast.Expr {
 			}
 		}
 	case token.COMMA:
+		firstComma = p.pos
 		// instance expression
 		args = append(args, index[0])
 		for p.tok == token.COMMA {
@@ -1549,6 +1551,11 @@ func (p *parser) parseIndexOrSliceOrInstance(x ast.Expr) ast.Expr {
 		return &ast.IndexExpr{X: x, Lbrack: lbrack, Index: index[0], Rbrack: rbrack}
 	}
 
+	if p.mode&ParseTypeParams == 0 {
+		p.error(firstComma, "expected ']' or ':', found ','")
+		return &ast.BadExpr{From: args[0].Pos(), To: args[len(args)-1].End()}
+	}
+
 	// instance expression
 	return &ast.CallExpr{Fun: x, Lparen: lbrack, Args: args, Rparen: rbrack, Brackets: true}
 }